For the last century, numerous researchers and inventors have come up with clever ideas to harness the solar irradiation in an efficient and inexpensive way to produce electricity. The pace of research has substantially increased during the last decades with high fossil fuel prices and awareness of the implications of releasing CO2 into the atmosphere.
The amount of solar irradiation impinging daily the earth is huge, yet the resource is feeble and constantly changing and despite best efforts, even the last commercial installations are still too expensive and have low inefficiencies. Notwithstanding the substantial price reduction of photovoltaic panels (“PV panels”), without inexpensive storage, PV panels do not offer a solution.
Solar thermal with thermal storage appears to be a better option, but the price is still too high to be competitive. Most solar thermal commercial operations utilize trough sun tracking parabolic mirrors, but in 2012, two large plants utilizing flat heliostats hitting a central receiver in a tower were installed in Ivanpah, Calif. and Crescent Dunes, Nev. A third option, utilizing a parabolic dish, with a Stirling engine at the focal point started operation in Maricopa, Ariz. in 2010, but filed for bankruptcy protection in 2012.
Fixed collectors cannot reach high temperatures even for a few hours and therefore are not used to generate electricity. Sun tracking parabolic trough consists of long lines of collectors, held horizontally, oriented North-South tracking the sun's movement from East to West. Unfortunately such arrangement suffers substantial cosine losses, especially in the winter. Inclining the collectors to ameliorate the cosine losses, poses insurmountable problems. The structure will have to be heavy and rigid to withstand wind, yet light so that the mirror could be moved to track the sun. State of the art plants require about 10,000 m2 per installed MW.
Heliostats are relatively flat mirrors with dual axis sun tracking. Each individual mirror moves independently, aiming to reflect sunlight into the central power. The particular cosine losses of each mirror depend on the position of the mirror with respect to the tower and the location of the sun. Overall, they are more efficient than the single axis sun tracking parabolic collectors, requiring about 7,000 m2 per installed MW, but since each mirror is only about 15 m2, it requires a large number of mirrors, each one with its own tracking controller and sensors. The Ivanpah's installation has 173,000 mirrors for a total area of 2,600,000 m2.
Dual axis sun-tracking paraboloid dishes have several advantages: normal direct irradiation is higher than horizontal irradiation (there are no cosine losses) and it is more evenly distributed both during the day and during the year. Overall, dual axis sun tracking paraboloid dishes could capture about 36% more solar energy on a yearly basis.
However, they have several disadvantages: (a) to capture meaningful amounts of energy the mirror needs to be large which adds complications to the sun tracking mechanism and offers more wind resistance; (b) to achieve high temperatures, a high concentration ratio is required, which implies a very accurate tracking mechanism, and; (c) the arrangement requires flexible yet leak proof connections for transporting the working fluid into and away from the focal point. After constructing a couple of demonstration units, the consensus reached was that the paraboloid dishes were not a very promising avenue. Mounting Stirling engines on top of a paraboloid dish simplified the tracking accuracy and the need of flexible connections, but difficulties with the Stirling engines failed to offer a competitive solution.
The present invention relates to the use of a dual axis sun tracking paraboloid dish collectors with a re-reflecting mirror above the focal point of the paraboloid dish, re-reflecting the concentrated light into an opening on the paraboloid dish, where the light is transmitted via light pipes to a cavity-receiver operating at high temperature. Several collectors clustered together could feed a single cavity-receiver and generate hundreds of kW.